Technical Field
The present disclosure relates to a low-consumption integrated magnetoresistor of an AMR (Anisotropic MagnetoResistance) type.
Discussion of the Related Art
Magnetic-field AMR sensors are used in a plurality of applications and systems, for example in compasses, in systems for detecting ferromagnetic characteristics, in the detection of currents, and in a wide range of other applications, by virtue of their capacity of detecting natural magnetic fields (for example, the Earth's magnetic field) and magnetic fields generated by electrical components (such as electrical or electronic devices and lines run by electric current).
As is known, the above magnetoresistive sensors exploit the capacity by appropriate ferromagnetic materials (referred to as “magnetoresistive materials”, for example the material known by the name of “permalloy” constituted by a Fe—Ni alloy) of modifying their own resistance in presence of an external magnetic field.
Currently, integrated magnetoresistive sensors are provided as strips of magnetoresistive material on a substrate of semiconductor material, for example silicon. During manufacture, the strip of magnetoresistive material is magnetized so as to have a preferential magnetization in a preset direction, for example the longitudinal direction of the strip.
In absence of external magnetic fields, the magnetization maintains the imposed direction, and the strip has maximum resistance. In presence of external magnetic fields having a direction different from that of preferential magnetization, the magnetization of the strip changes, as does its resistance, which decreases, as illustrated in FIGS. 1A and 1B.
In FIG. 1A, which shows the magnetization M in absence of an external magnetic field, a magnetoresistor 1 is formed by a magnetoresistive strip 2 having a longitudinal direction parallel to axis X (referred to as also “easy axis”, since this direction is the easiest strip magnetization direction). The magnetoresistor 1 is supplied with a current I flowing in the longitudinal direction of the strip. In this condition, the magnetization M is parallel to the easy axis EA.
In FIG. 1B, the magnetoresistor 1 is immersed in an external magnetic field Hy parallel to the axis Y (also referred to as “hard axis” since it is the direction in which magnetization of the magnetoresistive strip 2 is more difficult). In this condition, the external magnetic field Hy causes a rotation of the magnetization M by an angle α with respect to the current I, causing a reduction of the resistance of the magnetoresistive strip 2 according to a law correlated to cos2α.
To linearize the plot of the resistance R at least in an operating portion of the curve, it is known to form, on the magnetoresistive strip 2, transverse strips 3 (referred to as “barber poles”), of conductive material (for example, aluminium), arranged at a constant pitch and with an inclination of 45° with respect to the direction of the easy axis EA, as illustrated in FIG. 2.
In this situation, the direction of the current I changes, but not the magnetization M (the direction of which still depends upon the external magnetic field), and the resistance has a linear characteristic around the zero point of the external magnetic field. In this way, any possible magnetic fields directed along or having a component parallel to axis Y can be easily detected.
FIG. 3 shows a magnetoresistive sensor 4 including four magnetoresistors 1 of the type illustrated in FIG. 2, connected to form a Wheatstone bridge, with the transverse strips 3 arranged in an alternating way in each branch 4a and 4b of the Wheatstone bridge. The two branches 4a, 4b are connected at two input nodes 5, 6 across which a biasing voltage Vb is applied. In this way, the output voltage Vo existing between the intermediate nodes 7, 8 of the branches 4a, 4b is correlated to the existing external magnetic field Hy.
The above magnetoresistive sensors work properly provided that each magnetoresistor 1 is magnetized in the direction of the easy axis in absence of external magnetic fields and as long as the imposed magnetization M persists.
To maintain the imposed magnetization M, magnetoresistive sensors generally comprise a set/reset coil (designated by 10 in FIG. 4). The set/reset coil 10 enables execution of refresh operations, including repeated rapid magnetization steps in the desired direction. As may be noted from FIGS. 4 and 5, the set/reset coil 10, here in the shape of a square spiral, has stretches 11 that extend in a transverse direction, preferably perpendicular, to the longitudinal direction of the magnetoresistive strip 2, parallel to the easy axis EA. In the example illustrated, see in particular the cross-section of FIG. 5, the set/reset coil 10 is formed in a third metallization level M3, over the magnetoresistive strip 2. In turn, the magnetoresistive strip 2 is formed underneath a first magnetization level M1 and the latter forms the barber poles 3. The described structure is formed in an insulation structure 12 overlying a substrate 13, for example of silicon and forming with the latter an integrated chip 15.
In practice, during the refresh operations, the set/reset coil 10 is supplied with a current of a high value and generates a magnetic field B that, at the magnetoresistive strip 2, is parallel to the direction of the easy axis (see, for example, the document No. U.S. Pat. No. 5,247,278).
However, the refresh operation calls for high currents for the set/reset operations and thus involves a high consumption of the magnetoresistor 1. The consumption can be reduced by reducing the duration of the set/reset pulse, for example providing pulses having a duration of a few microseconds, repeated every 1-100 milliseconds. However, miniaturization of devices and integration of an ever-increasing number of functions in portable devices require a further reduction of the power consumption levels so that the reduction of duration of the set/reset pulses down to the values indicated above is not always sufficient.
Moreover, the high value of the set/reset currents is not easy to obtain. In fact, for this purpose, the driving circuit (generally provided as ASIC—Application-Specific Integrated Circuit) requires the presence of power MOS transistors that are able to carry the required high currents, with consequent increase in the overall dimensions and costs. In addition, the circuitry for generating the pulses requires the presence of an external capacitor. However, these requirements clash with the desire of scaling for small packaged sensors of reduced dimensions (e.g., 1.5×1.5 mm2). Manufacture of the capacitor as an external component on the application board involves further costs and implementation difficulties. Another problem linked to the presence of high pulses in the device resides in that these pulses may capacitively couple to the digital part, driving spurious resets thereon, with undesirable effects on operation of the device.
Another problem in known magnetoresistors lies in that, if they are immersed in a high stray field having the same direction as magnetization, and thus opposite direction as the field generated by the set/reset coil, the initial set/reset condition may be cancelled out, causing errors in the measurements. This problem involves a reduction of the maximum magnetic field that can be detected by the magnetoresistor. This reduction is particularly undesirable in the case of magnetoresistive sensors mounted in cellphones and other mobile devices, operating in environments where increasingly strong stray magnetic fields are present.